This invention relates to antennas and, more particularly, to reflecting antennas with concave reflectors.
The use of paraboloidal antennas for microwave transmission and reception is well known. Paraboloidal antennas are used because of directional attributes and high gains that occur at the focal point of the parabola-of-revolution. Omni-directional electromagnetic energy emitted at the focal point of a paraboloidal antenna will be reflected as collimated radiation. Similarly, electromagnetic energy traveling on an axis parallel to the axis of a paraboloidal antenna, such as a far field omni-directional or laser/maser source, impinging upon a paraboloidal antenna will be reflected to the focal point. The incoming electromagnetic energy is focused to a very compact focal point.
The general equation for a paraboloid is: z2/a2+y2/b2=x. A representation of such a paraboloid is shown in FIG. 1. Considering the plane where z=0 then y2/b2=x or y2=b2x and for such an equation the focus of the parabola in the plane where z=0 equals b/2. This focal point is the same distance for any of the planes containing the x-axis. The x-axis is the axis of symmetry.
The concentration of the received energy at the focal point is a good way of achieving high gains. The high gain region is located tightly around the focal point of the paraboloidal antenna. The tightness of that focal point also has some disadvantages. An installation with the axis of symmetry of the paraboloidal antenna not parallel to the incoming signal will cause a sharp signal drop-off if the angle between the axis of symmetry and the incoming signal increases. Similarly, high wind or icy weather can affect the effective gain of a paraboloidal antenna by deflecting the axis of symmetry from the direction of an incoming signal. Electromagnetic energy coming in to a paraboloidal antenna at an angle to the axis can be received just fine, or it can be just barely received depending upon the size of the angle. At approximately 15xc2x0 from the axis the gain drops from substantially similar to the gain at the focal point, to substantially zero. Such sharp differences in reception over such a relatively small angle is a problem for which antenna designers and antenna installers must allow. Considering that steel structures sway (some of the tallest buildings sway as much as 10 inches) in high winds, such sway alone could rule out use of a parabolic antenna on top of such structures.
The above problems are solved, and a number of technical advances are achieved in the art, by a concave antenna that is substantially paraboloidal but has a larger focal point so that the gain of the antenna does not drop so sharply with respect to the angle the incoming wave front makes with the axis of the antenna.
In accordance with an embodiment of the invention, a concave antenna having an axis along which at least two focal points are located is provided. Each of the focal points corresponds to a portion of a respective parabolic antenna having an axis along the concave antenna axis and a respective focal point along the concave antenna axis. Each respective axis is skewed with respect to the other axes.
In accordance with another embodiment of the invention, a concave antenna having at least two axes along which at least two focal points are located. Each axis is not co-linear with any of the other axes. Each of the focal points corresponds to a portion of a respective parabolic antenna having a respective axis and a respective focal point along the respective axis. Each respective axis intersects with respect to one of the other axes.
In accordance with another embodiment of the invention, a concave antenna having at least two axes along which at least two focal points are located. Each axis is not co-linear with any of the other axes. Each of the focal points corresponds to a portion of a respective parabolic antenna having a respective axis and a respective focal point along the respective axis. Each respective axis is parallel with respect to one other axis.
In accordance with another aspect of the invention, a concave antenna having at least two axes along which at least two focal points arc located. Each axis is not co-linear with any of the other axes. Each of the focal points corresponds to a portion of a respective parabolic antenna having a respective axis and a respective focal point along the respective axis. Each respective axis is parallel with respect to one of the other axes.